The present invention relates to the recovery of alkali metal fluorides such as NaF from slurries in organic ether solvents such as diphenyl ether.
Alkali metal fluoride salts exhibit low solubilities in water and extremely low solubilities in most organic media. Nevertheless, when such fluorides form a finely-divided solid phase in slurries of organic ether solvents, simple methods of separation (centrifugation or filtration) become difficult. There are generally two desired goals of such recovery: first, to recover as much of the solvent with a minimum of degradation in a form suitable for recycle, and, second, to recover the fluoride salt in a form that can be worked up to contain a minimum of organic contaminants.
In particular, in the process of U.S. Pat. No. 4,374,111 to Lefrancois (1983), a silicon halide such as silicon tetrafluoride is reacted with an alkali metal hydride such as sodium hydride in a diaryl ether solvent such as diphenyl ether (especially in admixture with biphenyl). The product, silane, is a gas which is easily removed (except for small amounts which remain dissolved in the solvent). The byproduct is a slurry of finely-divided fluoride in ether solvent, e.g. finely divided sodium fluoride in diphenyl ether/biphenyl. Usually some unreacted hydride and/or some alkali metal fluosilicate will also be present in the alkali metal fluoride phase. Such slurries have proved difficult to filter or centrifuge (as described below), with large amounts of solvent adhering to the particles in a sludge that usually prevents good phase separation or rapid and high-yield filtrate recovery. Both dissolved silane and unreacted sodium hydride present potential hazards in dealing with this sludge because of their reactivities with air and/or water.